This invention relates to the separation of sulfur-containing compounds from coal by froth flotation techniques.
Many coals contain relatively large amounts of sulfur ranging generally from less than one percent to as high as about 6 percent. Inorganic sulfur, which is predominantly in the form of pyrite (FeS.sub.2), generally accounts for about 40 to 80 percent of the sulfur in most coals. The inorganic sulfur is present in macroscopic and microscopic forms. The macroscopic form is generally present as veins, lenses, nodules or beds while the microscopic form occurs as finely disseminated particles which may be as small as one or two microns in diameter. The balance of the sulfur present in the coal is organic sulfur. The organic sulfur is typically present as mercaptans and sulfides and is incorporated into the coal structure itself.
Air pollution resulting from the burning of sulfur-containing coals is becoming of increasing concern due to the acid rain problems experienced in various parts of the world. The sulfur dioxide emitted when sulfur-containing coals are burned is thought to be a major factor in the acid rain problem. Various approaches to limiting the amount of sulfur dioxide emitted when sulfur-containing coal is burned have been investigated. One approach is to remove the sulfur dioxide from flue gases resulting from burning sulfur-containing fuels such as the process described in U.S. Pat. No. 4,612,175. Other approaches are directed to removing the sulfur from the coal before it is burned. Since the organic sulfur is typically extremely difficult to remove from the coal, the major portion of efforts in this area have been focused on the removal of the inorganic sulfur from coal.
One approach to the removal of inorganic sulfur from coal is flotation. Flotation is a process of treating a mixture of finely divided raw coal suspended in a liquid. Flotation allows the separation of the desired solid, coal, from undesired finely divided solids, gangue, such as pyrite and ash that are also present in the liquid. A gas is introduced into the liquid or created in situ to provide a frothy mass. This frothy mass will contain certain of the solids and carry these to the top of the liquid with the froth and leave other solids suspended in the liquid. Flotation is based on the principle that introducing a gas into a liquid containing different solid particles causes the selective adherence of some gas to part of the suspended solids and not to others. The particles adhering to the gas are lighter than the other solids and thus are floated to the surface while other particles not adhering to the gas remain suspended in the liquid. The selective adherence of the gas to some of the solid particles, but not to others, is due to physical, chemical or surface differences in the solid particles.
Coal is normally hydrophobic in an aqueous mixture That is, coal particles do not wet easily with water and therefore have some natural tendency to adhere to the gas bubbles. Various chemical additives are used in coal flotation to enhance the tendency of coal to float. Collectors, one type of these chemical additives, are commonly used to encourage the natural hydrophobicity of the coal. The collector increases the efficiency with which the gas bubbles attach to the coal. In situations wherein the coal is oxidized or otherwise difficult to float, a promoter may be added in addition to the collector to promote its efficiency. Another important chemical component, usually employed in coal flotation, is a frother which helps to control the rate and efficiency of the contact between bubbles and particles: of the adherence of the particle to the bubble; and of the removal of the bubble and particle from the liquid.
In addition to the use of chemical additives, a necessary part of any successful coal flotation process requires sufficient size reduction of the raw coal particles prior to actual flotation. The size reduction is necessary so that the majority of coal and the various gangue solids present exist as physically distinct particles (liberated particles) or as particles existing in a loose agglomeration Only when the particles exist in this state are the chemical additives discussed above successful in separating coal from gangue.
When the coal and the various gangue particles possess similar characteristics, it becomes difficult to separate them using simple flotation. When the differences in the characteristics of the solid particles are small or when the desirable and gangue solids both tend to float as is often the case with coal and pyrite in practice, it is necessary to use various methods to create or enhance the differences in the particles so that separation by flotation may be accomplished. Various techniques and processes exist to accomplish this.
One technique used to separate coal from inorganic sulfur-containing compounds in flotation processes focuses on the use of depressants to depress the flotation of either the coal or the inorganic sulfur-containing compound. A depressant is an agent that, when added to the flotation system, exerts a specific action on the material to be depressed thereby preventing it from floating. Various theories have been put forth to explain this phenomenon. Some of these include: that the depressants react chemically with the mineral surface to produce insoluble protective films of a wettable nature which fail to react with collectors: that the depressants, by various physical-chemical mechanisms, such as surface adsorption, mass-action effects, complex formation or the like, prevent the formation of the collector film: that the depressants act as solvents for an activating film naturally associated with the mineral: and that the depressants act as solvents for the collecting film. These theories appear closely related and the correct theory may eventually be found to involve elements of most or all of these and more.
U.S. Pat. No. 3,919,080 teaches that the flotation of inorganic sulfur as pyritic sulfur in the aqueous flotation of coal particles is depressed by the addition of sulfite to the flotation pulp. U.S. Pat. No. 3,807,557 discloses that pyrite is removed from coal in a two-stage flotation process. The conventional first flotation is followed by a second stage which uses an organic colloid as a depressant for the coal. The use of polyhydroxy alkyl xanthate depressants to depress the flotation of pyrite in coal flotation is taught in U.S. Pat. No. 4,211,642. Great Britain patent application 2,174,019A teaches that a compound which has one group capable of adhering to the surface of a hydrophilic mineral which group is bonded to a second group which is polar in nature and has hydrophilic properties is useful in coal flotation to depress the flotation of pyrite.
Many approaches have been suggested for the separation of inorganic sulfur from coal. However, the methods suggested are not without problems. Some of these problems include the removal of insufficient quantities of inorganic sulfur from the coal and lower overall coal recoveries. Thus, what is needed is a process for the separation of coal from sulfur that is inexpensive and simple to use and that significantly reduces the amount of inorganic sulfur remaining with the coal while not adversely affecting coal recovery.